There exists today a group of steels which are characterized by among other things enhanced mechanical properties including higher yield strengths and tensile strengths than plain carbon structural steels. These are known as high-strength, low-alloy (HSLA) steels. Different types of HSLA steels are available, some of which are carbon-manganese steels and others of which are microalloyed by additions of such elements as niobium, vanadium, and titanium to achieve enhanced mechanical properties. The original demand for HSLA steels arose from the need to obtain improved strength-to-weight ratios to reduce dead weight in transportation equipment. In addition to the original uses, HSLA steels are used today in a wide range of applications including vehicles, construction machinery, materials-handling equipment, bridges and buildings.
Commercial HSLA steels typically have minimum yield strengths of 40 to 50 ksi and minimum tensile strengths of 60 to 70 ksi. The mechanical properties and other characteristics of HSLA steels are set forth in standard specifications such as SAE J410c. Under these specifications, HSLA steels are produced to specified mechanical properties in their hot rolled condition. That is, the mechanical properties of HSLA steels depend on their hot rolled condition. These steels are not heat treated except for any annealing, normalizing or stress relieving done subsequent to cold reduction. Heat treated grades are available as plate, bars and, occasionally, sheet and structural shapes and depend on either precipitation hardening or quenching and tempering to develop the specified mechanical properties.
One grade of high-strength low-alloy steel under SAE J410c is grade 950 A,B,C,D, which is characterized by a minimum yield strength (0.2% offset) of 50,000 psi, minimum tensile strength of 70,000 psi, and minimum elongation (2-inch specimen) of 22%. This material exhibits its mechanical properties as hot rolled, and when later cold reduced to sheet thickness, is subjected to a low temperature recovery anneal for an extended period of time to maintain the as-rolled mechanical properties. This recovery anneal is disadvantageous because of either the extended times required for box annealing or the enormous investment required for equipment for continuous annealing.
There thus exists today a need for steels possessing the desired combination of strength and ductility required for HSLA steel applications but which can be produced economically from cold reduced sheet stock without the need for extended recovery annealing. Moreover, there exists a need for such steels wherein the higher mechanical properties, particularly yield strength and tensile strength, are achieved without the intentional inclusion of microalloying agents such as niobium, titanium and vanadium, which otherwise would add significantly to the cost of the steel.